1. Field of the Invention
This invention relates to schemes for protecting electric power distribution systems from the effects of current surges attributable to geomagnetic storms, and more particularly to the method and apparatus for reducing the effects of large direct currents induced in electric power equipment, for example, by geomagnetic storms.
Recently-published reports describe the problems that electric power distribution systems are experiencing as a result of direct currents (D.C.) induced in power lines by geomagnetic storms. Currents of as high as one million amperes are reported to circulate in the ionsphere around shifting paths as a result of solar flares. These induce potential differences as high as 20 volts per kilometer of separation between points on the Earth's surface. (See, for example, "Bracing for the Geomagnetic Storms," IEEE Spectrum, March 1990, pgs. 27-33.) As a result, large currents can flow in conductors of low impedance that connect the spaced points. Typically, the points may be connected by a three-phase transmission line having Y-connected transformers at each end with their neutral connections grounded.
A portion of the current attributable to the difference of potential at the spaced neutral grounding points can flow in each transformer winding of a transmission system and can easily cause half-cycle saturation in the transformer cores. This half-cycle saturation causes large harmonic currents, local heating, and other adverse effects which can precipitate system shutdown and regional power blackouts.